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Re: [TowerTalk] Balun Recommendation

Subject: Re: [TowerTalk] Balun Recommendation
From: Steve Hunt <>
Date: Tue, 17 Apr 2012 22:05:02 +0100
List-post: <">>

I'm not advocating bead chokes; I fully appreciate the benefits of "N^2" 
on a toroid core and the ability to put the SRF somewhere useful because 
of the increased stray capacitance. Rather, I am trying to understand 
why you would label a choke "And it's _Inductive!_ (Bad)", whose 
Resistive component is greater than its Reactive component over the 
large majority of the HF spectrum.

On a separate topic - one where I think we could agree - I recently made 
measurements on my ladderline-fed doublet to determine its 3-terminal 
equivalent circuit. This is a doublet which is supposedly balanced: 
equal length legs; ladderline coming away at right angles etc. On 80m 
the 3-terminal Star network equivalent turned out to be:

Z1 = 15 - j70
Z2 = 1.6 - j109
Z3 = 31 +j110

Compare those figures with a completely balanced and floating load, 
where Z1=Z2 and Z3=Infinity, and it's clear why we need to consider a 
choke even on a nominally "balanced" antenna.

Steve G3TXQ

On 17/04/2012 20:27, Jim Brown wrote:
> Now, here's where we diverge a bit on our analysis of the usefulness of
> strings of beads.  Yes, a single turn through #31 (that is, simply
> clamped onto the cable) shows resistance the same general order of
> magnitude as reactance between 4 MHz and 20 MHz, you've got to have a
> LOT of beads in that string to hit 5K ohms resistive, which is known to
> be a good design value to make a dent in RX noise. By contrast, it's
> easy to hit 5K with the multi-turn chokes on #31 or #43.  The only times
> I use a "string of beads" in single turn fashion are either at VHF,  or
> where it simply isn't possible to wind turns.

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